A bipolar junction transistor (BJT) typically has better current performance and speed than a metal oxide semiconductor (MOS) transistor and thus is widely used in analog, power radio frequency (RF) integrated circuit (IC) designs.
A BJT includes an emitter, a base, and a collector and can be classified as a vertical BJT and a lateral BJT, depending on the direction that charges emitted from the emitter move.
A BJT may be used for an electrostatic discharge protection device for protecting an internal IC.
FIG. 1 is a cross-sectional view of a related art lateral BJT used as an electrostatic discharge protection device. A BJT having an npn structure is shown in FIG. 1.
Referring to FIG. 1, the related art BJT is formed on a p-type semiconductor substrate 11. A first device isolation layer 14, a second device isolation layer 15, and a third isolation layer 16 are formed to distinguish an emitter area, a base area, and a collector area from each other.
A first well 12 is formed by implanting a p-type impurity into the semiconductor substrate 11 at the lower side of the first device isolation layer 14, and a second well 13 is formed by implanting an n-type impurity into the semiconductor substrate 11 at the lower side of the third device isolation layer 16. An area having the first well 12 becomes an emitter and base area, and an area having the second well 13 becomes a collector area.
Also, an n+ emitter 19 is formed by implanting an n-type impurity between the first device isolation layer 14 and the second device isolation layer 15, and a base 17 is formed by implanting a p-type impurity into the other side of the first device isolation layer 14.
Then, a collector 18 is formed at one side of the third device isolation layer 16 by implanting an n-type impurity on the surface of the second well 13.
When such a BJT is used fir an electrostatic discharge protection device, during a circuit design, an input/output (I/O) terminal is configured, a collector is connected to the I/O terminal, an emitter is connected to ground, and a base is connected to ground through resistance.
Such a BJT is used to protect a semiconductor device from electrostatic charge. That is, when an electrostatic voltage of more than 2000 V is applied to the I/O terminal, the BJT discharges the electrostatic current to ground instantly. Accordingly, in order to use a BJT as an electrostatic discharge protection device, parameters such as triggering voltage (Vt), holding voltage (Vh), and breakdown voltage (Vb) need to be satisfied in a range that does not invade an operation area of a core circuit and does not exceed a failure area of a core circuit.
However, a lateral BJT is not as effective as a vertical BJT in terms of electrostatic discharge protection performance.
Thus, a related art method of switching the positions of the emitter 17 and the base 19 is used as shown in FIG. 2. Referring to FIG. 2, when the positions of the emitter 17 and the base 19 are switched, the tunneling effect of a BJT occurs faster, so that this may advance the triggering point.
However, since it is difficult to obtain satisfactory electrostatic discharge protection performance only by switching the positions of an emitter and a base, the size of a BJT may be increased. However, this method also increases the volume resulting from the increased size.